Proteins are increasingly being used as pharmaceuticals, diagnostic agents, catalysts in biochemical processes, and growth promoters in mammalian-cell culture. Commercial processes involving the use of thiol-containing proteins are hampered by the lack of practical processes to manipulate the oxidation state of protein thiols. Although the oxidation state of protein thiols can be manipulated by thiol/disulfide exchange, conventional processes require adding an excess amount of expensive, potentially hazardous thiol reagent. This program is aimed at the development of a novel membrane-based thiol/disulfide-exchange process that overcomes these drawbacks. The process uses a catalytic amount of thiol reagent that is continuously regenerated using a membrane-based process. In Phase I, the investigator successfully demonstrated feasibility of the process, using it: 1) to reduce the disulfide linkage in oxidized glutathione, and 2) to reactivate essential thiols on an enzyme that had been inactivated by air oxidation. In Phase II, the investigators will:1) characterize the process, 2) demonstrate the applicability of the process to three target applications, 3) identify scale-up issues, and 4) demonstrate the technical and economic feasibility of the process to produce a target product at pilot-plant scale in preparation for Phase III commercialization.